This invention relates to a photo-semiconductor device, and more particularly to such a device in which dark current is reduced.
Photo-semiconductor devices such as photodiodes are usually used for light detection. Light measuring equipment is required to be able to detect even a weak light signal with high accuracy so that it is necessary to reduce the internal noise (i.e. dark current) of the photodetector to the lowest minimum possible. Especially, a spectrophotometer, for example, must treat light having a rather wide range of wavelengths covering visible and ultraviolet regions. However, an ordinary semiconductor light receiving element is usually less sensitive to ultraviolet light than to visible light. It is therefore necessary to reduce the dark current of a detector used for shorter wavelengths to a smaller value than the dark current of a detector used for longer wavelengths.
A typical type of conventional photodiode has a p.sup.+ nn.sup.+ or n.sup.+ pp.sup.+ structure. For convenience of explanation, a reference will be made to a p.sup.+ nn.sup.+ type photodiode. The dark current I.sub.D of the p.sup.+ nn.sup.+ photodiode is governed by the high resistance region side of the p.sup.+ n junction, i.e. an n-type layer serving substantially as an active region for light absorption. When the thickness of the n-type layer is much greater than the diffusion length of the minority carriers in the n layer, the dark current I.sub.D is given, as well known, by the following equation: EQU I.sub.D .perspectiveto.S.sub.q D.sub.p P.sub.n /L.sub.p ( 1),
where S designates the area of the junction, q the magnitude of the electronic charge (elementary charge), D.sub.p the diffusion constant of the minority carriers in the n layer, P.sub.n the number of the minority carriers in thermal equilibrium in the n layer, and L.sub.p the diffusion length of the minority carriers in the n layer. If the thickness of the n layer or active region is smaller than L.sub.p, the dark current T.sub.D takes a value smaller than that given by the equation (1).
Therefore, in order to reduce the dark current of a conventional photodiode, it is necessary to decrease the thickness of the active region or to increase the impurity concentration of the active region. For example, U.S. Pat. No. 3,534,231 has proposed a photodiode structure in which the dark current or bulk leakage current is reduced by making the n layer or active region so thin that a highly doped n.sup.+ layer may be disposed within the diffusion length of the p.sup.+ n junction. (In fact, the U.S. Patent shows an n.sup.+ pp.sup.+ structure.) Further, the U.S. Pat. No. 3,534,231 has also proposed a structure in which a p.sup.+ layer is substituted for the n.sup.+ layer and in which the dark current is reduced by substantially making the n layer or active region so thin that the p.sup.+ layer may be disposed within the diffusion length of the front p.sup.+ n junction. (In fact, the U.S. Patent shows an n.sup.+ pn.sup.+ structure.)
These approaches are indeed successful in the reduction of dark current, but the resultant structure causes the degradation of photo-electric conversion efficiency. Namely, the reduction of the thickness of the active region leads to the overall decrease in the absorption of light in the active region and also to the inefficient absorption of light having a small absorption coefficient. The absorption coefficient usually decreases with the increase in wavelength. On the other hand, the high impurity concentration in the active region makes it difficult to obtain a desired photocurrent level since the life time of carriers generated in the active region by the irradiation of incident light and contributing to a photocurrent is shortened. According to the above approaches, it is thus difficult to obtain a high photo-electric conversion efficiency over a wide range of wavelengths. Also, those approaches have a drawback that the inverse withstand voltage of the resultant device becomes low. Accordingly, there is a limit to the attempt to improve the device performance by merely controlling the thickness of the active region and/or the impurity concentration in the active region. Therefore, a drastic reformation of the light detecting element structure is desired to perform the light detection with a high S/N ratio while reducing the dark current without degrading the photo-electric conversion efficiency.
A multi-channel detection scheme using such conventional photodiodes has already found its application in image sensors and position detectors. With the improvement of the stability and sensitivity of photodetectors themselves based on the development of semiconductor techniques, the application of the multi-channel detection scheme to spectral photometry has been proposed. Since in that case the reduction of size and weight and the improvement of performance can be expected, a self-scanning photodiode array incorporating a scanning circuit therein using a charge coupled device with photodiodes as light detecting means is considered to be a promising example. Scientific instruments such as spectrophotometers etc. must detect a weak light signal with high accuracy. In those applications, the most important thing is how small a level of incident light can be detected while keeping a predetermined S/N ratio. The light detection with the predetermined S/N ratio encounters a problem that the minimum detectable limit of incident light depends on the level of dark current. With a light detecting device having a self-scanning function, the noise from the scanning circuit as well as the internal noise due to dark current must be taken into consideration. If some measures to reduce the former noise are made, the latter noise arising in the light detecting element (i.e. photodiode) used will determine the detectable limit level. It is therefore most important to reduce the dark current of the light receiving section.